Accumulating evidence suggests mitochondrial dysfunction and apoptosis contribute to Alzheimer's disease (AD) pathogenesis. Evidence also implicates presenilin proteins in mitochondrial alterations. Active ?-secretase and A (-amyloid peptide) formation have been detected in mitochondria, and presenilin 1 (PS1) and PS2 have been implicated in apoptosis. In an effort to understand presenilin's role in apoptosis, we identified a novel mitochondrial apoptotic protein PSAP that is associated PS1. Furthermore, our recent study shows that PSAP is specifically expressed in brain neurons, interacts with death receptor DR6, and forms a complex with pro-apoptotic molecules Bax and Bak upon TNFa induction of apoptosis. These findings provide a molecular link between PS1 and apoptosis. Interaction with PS1 also suggests PSAP may play a role in regulating ?-secretase catalyzed processing of APP and A formation. We hypothesize that PSAP is involved in both presenilin-catalyzed ?-secretase activity and presenilin-regulated apoptosis, specifically in the mitochondria. This hypothesis will be tested by two specific aims. Aim 1, determine how presenilin regulates PSAP-mediated apoptosis and how PSAP interferes with ?-secretase-mediated APP processing and A formation in vitro. We will first determine if the C-terminal fragment of PS1 (PS1C), which interacts with PSAP, affects TNFa-induced apoptosis, and specifically, whether PS1C interferes with TNF1-induced PSAP-Bax and PSAP-Bak complex formation and/or PSAP-DR6 interaction. We will also determine whether and how PSAP affects APP processing and A formation by overexpression or knockdown of PSAP in a cultured cell model. Aim 2, generate a PSAP knockout mice model to determine the physiological role and its importance in vivo. Since PSAP is a homolog of mitochondrial carrier proteins, which play an important role in mitochondrial function, we will first determine the effect of knockout of PSAP on animal development. We will also determine the effect of knockout of PSAP on TNFa-induced apoptosis and on the mitochondrial localization and activity of the ?-secretase complex both in vivo and in vitro. Significance: Accumulating evidence supports the idea that apoptosis and mitochondrial dysfunction play an important role in the pathogenesis of AD. However, the lack of a direct molecular link between presenilin and apoptosis has cast a shadow over these notions. Thus, the novel finding that the most disease-accountable PS1 is associated with an apoptotic mitochondrial protein provides a new avenue for studying the mechanism underlying molecular events of mitochondrial dysfunction and associated apoptosis that contribute to the pathogenesis of AD. Thus, the proposed study may lead to the identification of new therapeutic targets for the disease. PUBLIC HEALTH RELEVANCE: Alzheimer's disease is the most common form of dementia, and an estimated 5 million Americans are suffering from this disease. The accumulation of A initiates a series of neurotoxic events, which result in brain dysfunction and neuronal cells death. Evidence also suggested that apoptosis plays a key role in neuronal death observed in AD. Thus, the objective of this proposal is to determine the pathogenic role of a novel proapoptotic molecule PSAP in A formation and apoptosis may lead to revelation of new targets for therapeutic intervention and development of new prevention strategies.